Discontinuity of impedance along a signal transmission line can cause reflections and create noise in a signal. Termination impedance is used to match impedance at points in the signal transmission line. Placing a suitable termination impedance at a point on the transmission line at which the transmission line would otherwise have an impedance discontinuity can equalize the impedance across the transmission line and eliminate unwanted signal reflection.
Termination impedance is often provided by resistors, and in some applications the resistors are placed on the printed circuit board (PCB) on which a semiconductor die or package is mounted. However, resistors placed on a PCB may not remove all reflections, and the resistors can complicate wiring on the motherboard. On-die termination (ODT) is an approach in which termination resistors are placed on the semiconductor die instead of on the PCB. ODT can reduce the number of resistor elements and wiring on the PCB, as well as eliminate the reflections that PCB-based termination cannot.